Organic Matter Deters Weeds

By Charlie Nardozzi

Most gardeners know that adding organic matter to soils produces many benefits, such as increasing water retention and better nutrient uptake. Now, research from the USDA Agricultural Research Service suggest that organic farming practices, such as additions of compost, crop rotation, and decreased use of chemicals and tilling increases the amount and diversity beneficial soil microorganisms. The increased populations of beneficial microorganisms suppress the growth of many weeds, allowing for less cultivation or herbicide usage.

The next step in the research is to identify which microorganisms are suppressing which weeds so that farmers, nurseryman, and home gardeners can manipulate the kinds of microorganisms in the soils to inhibit problem weed growth. Tests so far have shown that bacteria such as deleterious rhizobacteria (DRB) feed on weed roots weakening the plant, making it less able to compete with crops and more vulnerable to disease and weeding. Bacteria also suppress weed seed germination. Some weed species that have shown to be effected by increased microorganisms numbers in the soil include giant, yellow, and green foxtail (Setaria) and amaranth.

Researchers are trying to isolate certain microorganisms such as DRB to create bio-herbicides that could attack hard to kill weeds such as velvetleaf and leaf spurge. For now, home gardeners should continue with organic gardening practices such as adding compost annually, rotating crops, growing cover crops, and avoiding the use of chemicals.